Visitors to a New Zealand oceanarium watched in astonishment when one shark tore a chunk out of another's stomach. But they were even more astonished when four baby sharks tumbled out of the gaping wound.

Staff at Kelly Tarlton's Underwater World, in Auckland, had no idea the female shark was pregnant until visitors came running to tell them about the impromptu caesarean section. Initially sceptical, they found the mother with a large cut and four little sharks swimming around.

The oceanarium's aquarist, Fiona Davies, said it was not uncommon for sharks to chomp on each other, even in the wild. But she had never heard of an incident like this. "It [the midwife shark] had to bite a certain part to let them [the babies] out, and do it without killing them or her [the mother]," she told the New Zealand Herald.

The relationship of the other shark to the mother was not immediately clear, but Ms Davies praised its actions, saying the unusual delivery had probably saved the babies' lives. Had the mother given birth naturally, probably at night, they would almost certainly have been eaten by adult sharks and stingrays.

As it was, four more babies were found inside the female when mother and young were removed to a nursery tank. The eight young sharks are now on show, along with some baby eagle rays, and will eventually be released into the wild.

The mother's wound has been stitched, and she is said to be doing "surprisingly well", given her rather radical birthing experience.